Traditionally, waste water quality monitoring was provided by lowering bottles, attached to a rope in the sewer to take samples. A few years ago, automatic samplers came on the market, driven by clocks and/or an external trigger. Today's machines, although often micro-computer driven, reflect their origins and are able to perform only very simple tasks. Typically they have probes that measure pH and temperature (parameters that appear in most by-laws on sewage), sometimes dissolved oxygen and conductivity; the output of these probes is either, via analog comparators, continuously compared to some preset threshold, or sampled through digital techniques for comparison with the digital equivalent of the mentioned thresholds.
To summarize the actual state of the art, water quality monitors can perform the following tasks:
Take a sample at a fixed interval by controlling a mechanical sampler; PA1 Read and sometimes record the values of one or more probes at a fixed interval; PA1 Take a sample when controlled by an external trigger (e.g. flow meter); and PA1 Take a sample and/or record the value when one or more of the measured parameters exceed some threshold. PA1 From 1 min. to 59 h 59 min. in 1 min. increments between consecutive samples; PA1 Non-uniform times in minutes or clock time; PA1 Random time interval between consecutive samples; PA1 From 1 to 9,999 flow pulses in single-pulse interval; and PA1 Flow paced in volume with attachable flow module.
A typical embodiment of a state of the art system is the ISCO (T.M.) family of samplers and flow meters. In the 6700 model, for example, the sample frequency can be selected as:
Optionally, one can add a pH module that can trigger samples when pH is outside a user provided range.
These systems are not well suited for long time base monitoring because, either they generate large amounts of monotonous data, or can ignore short term significant phenomena. Besides, the fixed threshold system can fill the sampler very fast, so that no bottles are available to receive interesting samples later on.